Devices for the contact-free measurement of temperature of the type under discussion have been known from practice for many years and are used to measure the temperature of a surface of a distant object. In the case of these measurements one utilizes the physical phenomenon that all surfaces with a temperature above absolute zero radiate electromagnetic waves because of molecular motion. This thermal radiation emanating from an object lies mainly in the infrared range and can be guided via infrared-sensitive imaging optics onto one or more infrared detectors. There the radiant energy is converted into electrical signals which can then be converted into temperature values based on the calibration of the detector. The temperature values measured can then be shown on a display, output as an analog signal, or represented via a digital output on a computer terminal.
The area of the object whose radiation is detected by the detector will be designated in general as a (radiation-) measured spot of the temperature-measuring device. In practical use knowledge of the location and the size of the measured spot is of the greatest importance for the precision and reliability of the temperature measurement. In that regard, the place and size of the measured spot are dependent on the construction and measuring ray path of the detector as well as on the special properties of the imaging optics. The curve of the size of the measurement spot as a function of the measurement distance also depends on the structure of the imaging optics.
In principle, long-range focusing can be distinguished from short-range focusing. In the case of long-range focusing, the detector is imaged at infinity and in the case of short-range focusing on a focus plane at a finite distance from the detector. For both systems, different sighting devices for visualizing the measured spot are known. In those devices optical markings are produced in the center of the measured spot for the marking of the precise position of the measured spot or, additionally or alternatively, along the outer circumference of the measured spot for marking the size of the measured spot.
Known from DE 196 54 276 A1 and U.S. Pat. No. 6,234,669, is a device for the contact-free measurement of temperature with optics imaging at a finite distance. There, several sighting rays are provided which are directed skew to the optical axis and aligned to each other in such a manner that each sighting ray, both in front of and behind, a focus point measured spot can be used to mark the size of the measured spot. The sighting rays are generated by a light source and diffractive optics disposed behind the light source, for example, in the form of a hologram. Here it is disadvantageous that for the generation of the plurality of sighting rays elaborate diffractive optics is necessary. The degree of efficiency of diffractive components of this type and their imaging quality is limited.
It is also a known practice to visualize the measured spot with a marking which is perceptible as a line completely encircling the measured spot. A continuous encircling of the measured spot is, for example, perceptible when a laser ray is guided with a rapidly rotating mirror around the measured spot, as is disclosed, for example, in U.S. Pat. No. 5,368,392. Due to their consumption of energy and an increased susceptibility to interference, movable mechanical components are disadvantageous in the framework of contact-free temperature measurement, in particular with the use in mobile infrared temperature measurements.